Gear Review: The North Face Mercurial Sleeping Bag Liner

Crank up your bag's temperature rating by 10 degrees with this liner.

Add 10°F

Got a bag you like already, just need a little more warmth? Give it a boost with the Mercurial Liner—a wide, semi-rectangular insert that also works as a standalone bag for warm summer nights. “The Liner was a trip-saver when a late-summer thunderstorm swept in chilly air,” said a thankful tester after a trek in the mountains above Snowmass, Colorado.

The sack is made of fleece lined with nylon, and it can be used nylon-side-in (adds about five degrees of warmth, but feels clammy against the skin) or fleece-side-in (for a 10-degree boost). The long zipper goes halfway around the bag’s perimeter and adds a few ounces to its overall weight, but it also enables fine-tuning warmth and quilt-style use. By itself on warm nights (think East Coast and Southwest summers), the Mercurial is far more comfortable than a stuffy insulated bag.

“Feels more like my bed sheets back home,” says one tester. The nylon shell doesn’t repel moisture, but the material dries almost instantly. A fleece-lined stuffsack transforms into a pillow when turned inside-out and stuffed with clothes. Packed up, it’s about the size of a roll of paper towels. Caveats: It’s heavy, and we wish it came in a mummy shape, for a better fit inside our favorite bags. $99; 1 lbs. 10 oz; 50°F